Although public attention has focused largely on Lyme disease, an array of spirochetal, protozoan, viral and rickettsial pathogens concurrently infect residents of sites in the eastern US in which deer are abundant. The abundance of deer and that of the vector arthropods that depend on this host recently have increased. Several of these deer-associated pathogens cause neurologic disease. Our objective, therefore, is to describe the frequency of meningitis and encephalitis caused by these infections, both individually and in combination, and to determine whether the resulting spectrum of disease may reflect a synergy between pathogens. In particular, we shall: (1) Determine how frequently meningitis and encephalitis are caused by deer-associated pathogens in residents of the northeastern U.S. and how frequently these pathogens occur in combination. Although we shall focus on the agents of Lyme disease, babesiosis, human granulocytic ehrlichiosis and TBE-group arboviruses, we shall seek to document the presence of other tick-borne microbes and the frequency of infection by such deer-associated, mosquito-transmitted agents as Jamestown Canyon virus, Cache Valley fever virus, and LaCrosse encephalitis virus, as well as two other important neurotropic viral species: enteroviruses and Herpes simplex virus. We shall determine prospectively how frequently meningitis and encephalitis is caused by deer-associated pathogens by means of a case-finding study. (2) Estimate the burden on human health imposed by these deer-associated pathogens and determine whether coinfection synergizes pathogenesis. In particular, we shall determine the outcome following meningitis or encephalitis caused by these deer-associated pathogens and whether coinfected people suffering from meningitis or encephalitis experience a greater severity of acute illness, a longer duration of illness or greater neurologic sequelae than if only one such pathogen were present. A cohort of patients experiencing meningitis or encephalitis will be enrolled in a prospective longitudinal study. This proposed effort will help define the health burden of neurological disease imposed on people residing in proximity to deer and will contribute to diagnosis and case-management of meningitis and encephalitis in people living in these areas.